In well-known metallurgical flotation methods and flotation machines, particles that are rendered hydrophobic are foamed from an aqueous slurry that contains these particles. The flotation machine includes a flotation cell and a rotor that is inside the flotation cell. The rotor is rotated at a mixing power, which maintains a suspension in the slurry and the mixing of air with the slurry, to form the foam. It is further known that the mixing power can be adjusted by adjusting the rotation speed of the rotor. An electric motor is normally arranged to rotate the rotor, and the rotation speed of the motor can be changed by means of a frequency converter that serves as an adjusting device.
When using the flotation machine, so-called sanding occurs; that is, solid matter accumulates on the bottom of the flotation cell, in the corner region between the bottom and the wall. Excessive accumulation of solid matter is not allowed, because the solid matter thus accumulated tends to block the inlet and outlet openings of slurry that are usually located in the area of accumulation.
In prior art, the matter has been solved in a simple way by selecting “an adequate amount” of mixing power; in other words, the rotor is rotated continuously at a high enough rotation speed, at which the solid matter does not accumulate excessively. This has often resulted in considerable excess power with respect to what is needed and, at the same time, in poor energy efficiency.
It has been observed that, in many cases, the metallurgical results do not deteriorate, even if the mixing power is reduced, but at some stage, a limiting factor of reducing the mixing power will consist of the excessive accumulation of solid matter, or sanding, on the bottom of the cell.
The energy efficiency is a very significant matter in flotation plants. For example, in a flotation plant, there can be 50 flotation machines in a series, each having a 300 kW electric motor. When each of them is continuously operated with excess power to prevent sanding, we are talking about a considerable item of expenditure.